The invention relates to a method for testing an electromagnetic flowmeter having a measuring tube and a coil arrangement for generating a magnetic field perpendicular to the direction of flow through the measuring tube, the current direction in the coil arrangement being periodically changed. The invention also relates to an electromagnetic flowmeter arrangement having a measuring tube, a coil arrangement for generating a magnetic field substantially perpendicular to the direction of flow through the measuring tube, an electrode arrangement substantially perpendicular to the direction of flow and to the magnetic field, a supply system for the coil arrangement, which has a current direction reversal arrangement, and a testing device.
A method and a flowmeter arrangement of that kind are known from GB 2 309 308 A. Here, testing is carried out by interrupting the normal connection between the measuring tube, that is, the electrode arrangement, and the coil arrangement, and by linking up an external measuring loop. During testing, a normal measurement is not therefore possible. There is also the danger that the interruption of the circuit and the subsequent connection will again cause errors that will not be identified. Testing is effected in that the ohmic resistance of the coil arrangement is determined by applying a voltage to the coil. As soon as the ohmic resistance is known, the voltage is interrupted and the inductance of the coil arrangement is determined by monitoring the decay of the current.
U.S. Pat. No. 5,639,970 describes a current selecting circuit for an electromagnetic flowmeter. This circuit is able to select the correct current and the correct frequency in dependence on the chosen flowmeter. The decision is made by monitoring the response of a coil to energization at relatively high frequency. The faster the signal response is, the greater can be the current through the coil arrangement.
The invention is based on the problem of enabling the flowmeter to be tested in the simplest possible manner.
That problem is solved in a method of the kind mentioned in the introduction in that after the change in the current direction at least one parameter of the current rise is determined and this is compared with a reference value.
The coil arrangement forms an inductor. In an inductor, the current cannot xe2x80x9cjumpxe2x80x9d. After the direction reversal, it therefore needs a certain time until it is again at its set value. The rise in the current is a kind of xe2x80x9cfingerprintxe2x80x9d for the corresponding flowmeter. As long as the flowmeter can operate undisturbed, that is, free from defects, the rise characteristics are practically identical with a very small range of variation range. Only when an electrical or magnetic defect occurs will the rise characteristic change. This is then an indication that the flowmeter is possibly supplying inaccurate measurement results and must be tested or exchanged. This construction has the advantage that both the electrical property and the magnetic properties are tested, since both electrical and magnetic influences have a characteristic effect on the rise of the current.
Testing is preferably carried out during measurement of a throughflow. Measurement of the throughflow does not even have to be interrupted, yet it is nonetheless possible to carry out testing virtually continuously or permanently. This has the advantage, moreover, that the flowmeter is tested precisely in the situation in which it is also operative.
In this connection, is it preferred that the reference value is determined at the flowmeter itself at an earlier time. At a specific time, therefore, for example, on commissioning, the desired parameter is established and this is stored as reference value so that it is available for future testing procedures. Each flowmeter therefore receives an individual reference value, so that testing can be carried out very accurately. Defects that can result from an erroneously pre-set reference value are virtually unknown.
The time period that elapses between two predetermined current values is preferably used as parameter. Since the rise in the current conforms to a predetermined physical natural law, as a rule an e-function, it is sufficient to determine the rise time between two values to obtain reliable evidence of the current rise per se.
Alternatively, or in addition thereto, in a further preferred construction the time period that elapses between change-over of the current direction and reaching a predetermined current value can be used as parameter. The time of change-over is identifiable very exactly. For example, the change-over signal can also be used as a trigger signal for a time-counter. The predetermined current value can be, for example, close to the maximum current value, that is, close to the current that exists in continuous operation. A relatively long period of time is therefore available, so that the testing can produce a correspondingly accurate result.
After change-over, a stepped-up voltage is advantageously used. This voltage, also known as the xe2x80x9cboostxe2x80x9d voltage, accelerates the build-up of the magnetic field, and consequently enables the actual measurement to be performed more quickly again. It does also change the current rise, but if the current rise is always carried out in the same manner, that is, with the same increased or xe2x80x9cboostxe2x80x9d voltage, the characteristic of the current rise can also be used here for testing.
Advantageously, the supply voltage of the coil arrangement is regulated ratiometrically in relation to a reference voltage, which is also used to determine the parameter. Voltage fluctuations cannot therefore have an adverse effect on the test result. The characteristic of the current rise is then the same, regardless of possible voltage fluctuations, which, of course, if possible, should not occur at all.
As an alternative or in addition to the above-mentioned parameters, the curve shape of the current rise can be used as parameter. This does increase the complexity of the testing, but allows even more reliable results.
In this connection it is preferred that the curve shape is formed by current values ascertained at predetermined times. These current values can be converted, for example, into digital signals that are evaluated in a microprocessor. The microprocessor can then compare the curve for the measured build-up of the coil current with one or more reference curves. The entire waveform can therefore be monitored. A curve shape varying from the target curve enables one to draw conclusions as to whether there is a variation in the magnetic circuit or in the electric circuitry.
Preferably, current rises following directly one after the other are compared with one another. In this way, information as to whether the build-up of the magnetic field is proceeding symmetrically is additionally obtained.
The problem is also solved by an electromagnetic flowmeter arrangement of the kind mentioned in the introduction, in that the testing device comprises means which, after a change-over of the current direction, determine at least one parameter of the rise in the current in the coil arrangement and compare it with a predetermined value.
As explained above in connection with the method, the rise in the current after change-over of the current direction in the coil arrangement is a significant feature of each flowmeter arrangement. As long as the flow meter arrangement does not change, this feature also remains unchanged. Variations point to a defect or at least to an inaccuracy. When the rise or a parameter depending thereon is compared with a given value, defects can be identified reliably and above all in good time.
The testing device preferably comprises a time-counter, and a rise time serves as parameter. Although in this case just a single variable is determined during each current rise, this is sufficiently reliable to allow credible testing or monitoring.
The testing device preferably comprises a comparator, which compares the current or a variable derived therefrom with a given value and which is connected to the time-counter. The comparator therefore triggers the time-counter whenever the current (or rather a voltage associated therewith) reaches a fixed given value. The time-counter then stops counting and has then, as it were, determined the length of time that the current has needed for its rise.
The time-counter is advantageously connected to a checking unit, which produces an error message whenever the time ascertained differs by more than a predetermined difference from a given value. An exact conformity of the rise time will be reached only in the rarest cases. A small tolerance range is allowed. If, however, the individual times lie outside this tolerance range, a defect is identified.
In series with the coil arrangement there is preferably arranged an electrical resistance, the temperature-dependent resistance behaviour of which is inversely proportional to that of the coil arrangement. Thermal influences on the coil current can consequently be compensated. The testing can therefore work within a relatively large temperature range with greater accuracy.
Preferably, a supplementary voltage supply system is provided, which is connected to the supply system by way of a change-over switch. After the change in current direction, first of all the supplementary voltage supply system with a higher voltage is used to build up the coil current. Return to xe2x80x9cnormalxe2x80x9d supply voltage is effected only when the coil current has reached a predetermined value. In that case, change-over to the supplementary voltage supply system can also be used as a starting time for the time-counter.
It is also preferred that the arrangement comprises an analogue-to-digital converter, which determines the analogue values in relation to a reference voltage, the value of which is also used as starting point for determining coil current and coil supply voltage. A constant relationship between the reference voltage of the analogue-to-digital converter, the coil current and the coil supply voltage can consequently be achieved. In this way, high measuring accuracy is achieved, without unduly high demands being made in respect of stabilising of the reference voltage, the coil current or the coil supply voltage.